1. Field of the Invention
The present invention relates to a gas sensor having a gas sensor element that detects the concentration of a specific gas, such as NOx, contained in a combustion gas or an exhaust gas of, for instance, a combustor, an engine, and the like, as well as to a method for manufacturing the gas sensor.
2. Description of Related Art
There is a need for further reduction in the amount of nitrogen oxide (NOx) contained in exhaust gas as regulations on the exhaust gas of an engine, such as an automobile, became more strict. In this regard, an NOx sensor capable of directly measuring the concentration of NOx has been developed.
The NOx sensor has a gas sensor element with one or a plurality of cells, each of which has a pair of electrodes formed on the surface of an oxygen-ion-conductive solid electrolyte layer, such as zirconia. A first pumping cell evacuates oxygen in a first measurement chamber while remaining in mutual communication with a second chamber for receiving the gas to be measured. Further, an oxygen concentration detection cell measures the concentration of oxygen in the first measurement chamber and controls the first pumping cell in such a way that a predetermined oxygen concentration is achieved within the first measurement chamber. Moreover, the gas to be measured having a controlled oxygen concentration flows into the second measurement chamber from the first measurement chamber. NOx contained in the gas to be measured is decomposed into N2 and O2 by applying a given voltage to the second pumping cell. A second pump current flowing between a pair of electrodes of the second pumping cell is measured, whereby the concentration of NOx in the gas to be measured is detected.
In such an NOx sensor, an inner second pump electrode of the second pumping cell is provided in the second measurement chamber and has the primary function of pumping the O2 component originating from decomposition of a trace amount of NOx contained in the gas to be measured. In order to enhance durability of the inner second pump electrode, a technique has been developed using, as an electrode material, cermet consisting of a Pt—Rh alloy and ceramic (see Patent Document 1). A technique for using, as a material for the inner second pump electrode, a mixture consisting of Pt—Rh powder and zirconia powder has also been developed (see Patent Document 2).    [Patent Document 1] JP-A-11-183434    [Patent Document 2] JP-A-2005-283240
3. Problems to be Solved by the Invention
The second pumping cell has a primary function of pumping O2 originating from decomposition of a trace amount of NOx gas. The concentration of NOx gas to be pumped is equivalent to several microamperes in terms of a second pump current. Therefore, even in a case where the pumping capability of the second pumping cell is low, the pumping capability is sufficient for decomposing the trace amount of NOx gas at the time of measuring NOx concentration.
In the meantime, at the time of heating the NOx sensor to activate the solid electrolytes of the sensor element so that they are capable of transporting oxygen, it is necessary to forcefully pump out a high concentration of O2 present in the second measurement chamber. In this regard, the pumping capability of the second pumping cell becomes deficient. Consequently, a problem arises in that there is an increase in the length of the so-called light-off time, namely, a duration from the beginning of start-up control of the sensor until the concentration of O2 in the second measurement chamber (the second pump current) is reduced to a given level by pumping.